This invention relates to an image recording apparatus such as a printer, copying apparatus or a composite image recording apparatus, and more particularly, it relates to the heat and pressure fixing of particulate thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to apply pressure and elevate the temperature of the toner to a point at which the constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to effect heating of the toner images within the nip. A large quantity of heat is applied to the toner and the sheet bearing the toner image. This heat vaporizes the moisture contained in the sheet. Since the heat quantity applied to the front side of the sheet is not equal to that applied to the backside thereof, the amounts of the evaporation are not equal. This results in sheet curling.
High toner mass area (TMA) copies, especially colored ones where large solids in excess of 2 mg/cm.sup.2 are not uncommon, acquire a degree of curl that is very difficult to live with. Various decurling structures and methods have been considered, but the solutions are at best a compromise, since copies are not usually uniformly covered with toner. Thus, with known decurling systems, decurling that is successful in an area having a high TMA would result in too much decurling in an area where there is a less toner coverage.
The problem is generally recognized as continued expansion of a paper substrate as it equilibrates with the moisture in the environment after the toner image is set in its dimensions.
Examples of structures and methods of curl prevention or decurling are as follows:
Laid-Open Japanese Patent Application No. 81270/1982 discloses, in order to solve the problem described above, that the sheet itself is improved from the standpoint of reducing the amount of curling. More particularly, the moisture content of a sheet is maintained 5.5-6.5% so as to maintain a desired rigidity in the sheet. This, however, limits the sheets which are usable, and can not satisfactorily be used with plain paper. In addition, the sheets are vulnerable to changes in ambient conditions, for example, the sheets absorb moisture under relatively high humidity conditions, resulting in less rigidity. Furthermore, when the amount of evaporation increases, the amount of curling is usually increased.
U.S. Pat. No. 4,375,327 relates to a heat fixing device including a heat fixing roller and a pressing roller, wherein a separation pawl is provided so as to contact the pressing roller, and the distance between the separation pawl and the nip between the fixing roller and the pressing roller is reduced.
Laid-Open Japanese Patent Application No. 113637/1976 discloses that the speed of sheet transportation be higher after the sheet is discharged from the nip in the fixing device than during its transportation in the fixing device.
The foregoing are not able to deal with the curling of a particular portion of the sheet or the entirety thereof. Particularly in the case of the duplex image recording, where the trailing end of the sheet in the first side printing becomes the leading edge in the second side printing, and therefore, it is necessary to regulate the curling in the neighborhood of the sheet trailing edge in the first side printing. The above-described devices are not satisfactory to meet this necessity. The '327 patent mentioned above is effective to regulate a wave-form curling, and the Japanese Laid-Open Patent Application can regulate curling only when the sheet is between the fixing roller and transporting means located downstream thereof. Thus, neither can deal with curling in the neighborhood of the leading or trailing edge of the sheet.
There are other proposals for the purpose of regulating or eliminating curling of the sheet at the discharge of the fixing roller, but they are not satisfactory in order to meet curling from the leading edge to the trailing edge.
Laid-Open Japanese Patent Application No. 143339/1978, noting that the sheet curling is the most particular problem in duplex printing, proposes that a toner image on the first side is temporarily fixed, and the toner images on the two sides are fixed to a sufficient extent when the second side toner image is fixed. During the fixing operation for the first side, that is, the temporary image fixing, the sheet does not receive a large quantity of heat so that the amount of curling is not large, but correspondingly, the image fixing is not sufficient. Those insufficiently fixed toner images may be scraped off by transporting rollers and so on during the sheet transportation for the second side image formation. As will be understood, the conditions for providing satisfactory fixing and the conditions for reduced curling, are contradictory to each other, and the conditions under which both are satisfactory are so limited that reducing the amount of curling by degrading slightly the fixing performance is practically very difficult.
Laid-Open Japanese Patent Application No. 24862/1984, discloses a water pan located above the fixing mechanism. The pan is heated by the heat emitted from the fixing mechanism after it is sufficiently heated. Thereafter, the water in the pan is heated thereby and is evaporated. In this system, a state is formed outside the copying mechanisms, wherein the water vapor is dispersed in the ambient atmosphere. Then, the sheet after being subjected to the image fixing operation is introduced in the ambience.
As will be appreciated, it requires a long time from the start of the water pan heating to the water therein becoming evaporated. The time is much longer than the time required from actuation of the main switch to the copyable state being reached. Therefore, the sheet curling could not be prevented during a significant time period from the time the copyable state is reached. Since the amount of the water contained in the water pan is necessarily limited, curl prevention is not performed after the water is consumed. It would be possible to replenish the water into the water pan, but this would require complicated mechanisms for the control because the temperature in and of the copying machine is increased by the continuous long operation.
U.S. Pat. No. 4,652,110 discloses an image forming or recording apparatus provided with an image fixing device of a heating type, which may cause curl of the recording material when the recording material is discharged from the fixing means. The causes of the curling are investigated, and a device is provided which can effectively correct or remove the curl of the recording material with a very simple structure. The device includes a passage for applying water vapor to the recording material, immediately after it is discharged from the fixing device. As a supply of the vapor, the moisture contained in the recording material is utilized.